Download The Heliocentric Universe

Lecture 4
The Heliocentric
Universe
September 16, 2015
1
2
Aristarchus (310-230 BC)
• Proposed Earth orbited the Sun.
• More easily explained retrograde motion.
• Animation
3
The direction of retrograde motion for a planet
as seen by an observer on Earth is
A. west to east relative
to the background stars.
B. east to west relative
to the background stars.
C. west to east relative
to the local horizon.
D. east to west relative
to the local horizon.
44%
45%
6%
A.
B.
C.
6%
D.
4
Aristarchus (310-230 BC)
• His hypothesis was rejected
– Earth does not feel like it is moving.
– No parallax of stars was observed.
5
Parallax
• If you look at an object from two different
places (but at same distance) it will appear to
move with respect to the background.
• Change in position = parallax angle
6
Parallax
• The greater the distance,
the smaller the parallax
angle
• The greater the baseline
(distance from A to B), the
greater the parallax angle.
baseline (km)
parallax angle ()  57.3 
distance (km)
or
baseline (km)
distance (km)  57.3 
parallax angle ()
Distance
Baseline
7
If you increase your baseline to twice its original
length, what happens to the observed parallax angle?
A. It increases to 2 times its88%
original angle
B. It decreases to ½ its
original angle
C. It increases to 4 times its
original angle
D. It decreases to ¼ its
original angle
A.
6%
B.
6%
C.
1%
D.
8
What is the distance to a star that has a parallax angle
of 0.580 arcsec when the baseline is 3.00×1015 km?
A. 1.74×1015 km
B. 5.17×1015 km
C. 1.07×1021 km
D. 2.77×1013 km
93%
km
2.
77
×1
01
3
km
1%
1.
07
×1
02
1
km
3%
5.
17
×1
01
5
1.
74
×1
01
5
km
3%
9
What is the distance to a star that has a parallax angle
of 0.580 arcsec when the baseline is 3.00×1015 km?
1.74×1015
A.
km
B. 5.17×1015 km
C. 1.07×1021 km
D. 2.77×1013 km
Convert to degrees:
1 deg
0.580 arcsec 
 1.6110 4 
3600 arcsec
15
57.3
3.0

10
km 

57.3  b
r


1.6110 4 
 1.07 1021 km
1000 m
1 ly
21
1.07 10 km 

km
9.46 1015 m
 113 106 ly
10
•Parallax was not
observed because the
stars are too distant
•Geocentric Model
becomes accepted.
11
Nicholas Copernicus (1473-1543)
• Geocentric model had
become complicated.
– Based on naked eye
observations.
– Small inaccuracies in the
past had become large
• Copernicus proposes
heliocentric (sun
centered) model of the
Solar System.
13
Basic Heliocentric View
• Heliocentric Model =
sun at center
Celestial Sphere
• All planets orbit the
sun, inner planets
faster than outer
• Moon orbits the
Earth.
S
• Circular orbits
J
• Animation
E
S
V
M
M
14
The Heliocentric Model
• Could predict positions of planets
– BUT only as well as Ptolemy’s model
• Could calculate distances to planets in units of
distance between the Earth and the Sun
– BUT the results could not be checked.
• Why did it gain acceptance? – It was simpler!
– Occam’s Razor: If choosing between competing
theories that are all similarly accurate, choose the
simplest one.
15
Copernicus’ model for the planetary system
A. placed earth at the center of the solar system but eliminated
epicycles.
B. placed the sun at the center of the solar system and
could
54%
predict the locations of planets more accurately than ever.
46%
C. placed the sun at the center of the solar system and could
calculate planetary orbit distances for the first time.
D. placed earth at the center of the solar system and was the first
to postulate that planets moved in epicycles.
0%
A.
0%
B.
C.
D.